1. Field of the Invention
The present invention relates to a semiconductor device and, more particularly, to a semiconductor device having a monocrystalline insulating film on a semiconductor substrate.
2. Description of the Related Art
The thickness of a gate insulating film in a field effect MOS device formed in a monocrystalline semiconductor substrate is becoming increasingly smaller along with recent trends toward a higher operation speed and a higher integration density of such devices. When the thickness of a gate insulating film is decreased, the threshold voltage is lowered, and the operation speed increases corresponding to the decrease in threshold voltage. The AC characteristics of the device can be particularly improved. Taking an EEPROM or the like as an example, the application conditions of the device become severer while the device is patterned finer. In this case, an oxide film formed by a conventional method no longer provides sufficiently high reliability.
Although a MOS device is micropatterned, not much effort for attaining improvements in the properties of an oxide film itself are made. For this reason, the power supply voltage is not so lowered. Under these circumstances, a high electric field is applied to a gate oxide film particularly during operation of the device. Electrons/holes generated from a channel region by impact ionization or the like are injected into an oxide film depending on boundary conditions such as the polarity of a gate electrode and the drain voltage. Such carriers are trapped in the oxide film to not only degrade long-term reliability but also lower the breakdown voltage or the like.
On the atomic level, when a high electric field is applied to a silicon oxide film formed by, e.g., thermal oxidation, Si-O bonds constituting a silicon oxide film network interact with an external high voltage. As a result, the bonds are dissociated, and the trap centers in which electrons or holes are trapped are formed. Subsequently passing electrons and holes are trapped by the trap centers, and the field strength distribution in the direction of film thickness is locally set higher than the average field strength, finally resulting in dielectric breakdown.
To solve this problem, a technique using a monocrystalline gate insulating film is recently proposed. For example, a monocrystalline cerium oxide (CEO.sub.2) is formed on a silicon (111) plane, as reported in J. Appl. Phys., vol. 69 (12), p. 8313 (1991). Monocrystalline calcium fluoride (CaF.sub.2) is grown on monocrystalline silicon, as reported in J. Appl. Phys. Suppl., Vol 21-1, p. 187 (1982).
Many of these techniques, however, are still under development or in the theoretical stage. In addition, some problems on calculation techniques as a principle of these techniques are left unsolved. Taking an oxide film as an example, a monocrystalline structure of a gate insulating film using an oxide film has not been accurately explained. Reports concerning the structure of this monocrystalline oxide film include the following:
One is M. Hane. et. al., "Atomic and Electronic Structures of an interface between silicon and .beta.-cristobalite", Physical Review, B. Vol. 41, No. 18, pp. 12637-12640 (1990). In this report, the .beta.-cristobalite is used as a monocrystalline gate oxide film, and a stable structure required for forming this on an underlying Si substrate is calculated. In this reference, however, initial layouts for the angles and distances of Si-O-Si or O-Si-O are incorrect.
W. A. Tiller, "On the Kinetics of the Thermal Oxidation of Silicon, III", J.E.C.S., Vol. 128, No. 3, p. 689 (1981) describes similar monocrystalline oxide film made of quartz. This reference describes the positional relationship between the underlayer and the monocrystalline oxide film made of quartz. However, the structure of the monocrystalline oxide film is simplified, and the principle of this reference is not necessarily correct.
More specifically, as for .beta. low-temperature cristobalite, J. R. Chelikovsky, et. al., "Electron states in a quartz", Physical Review, B. Vol. 15, No. 8, pp. 4020-4029 (1977) points out that "many papers describe models in which the angle of Si-O-Si is simply set to 180.degree. and oxygen atoms are regularly located between the atoms of a diamond lattice, but these models are not accurate and often result in wrong conclusions". Judging from this description, the above papers in 1990 and 1981 are based on the errors pointed out by J. R. Chelikovsky, et. al. For this reason, it is not too much to say that the problem analysis is retrogressive.
The same drawback as in the gate insulating film described above equally applies to a tunnel insulating film in a semiconductor device in which information is stored by accumulating or removing a charge, through the tunnel insulating film, in or from a floating electrode surrounded by an insulating film. More particularly, when a high electric field is applied to the tunnel insulating film, the Si-O bonds constituting a silicon oxide film network interact with an external high voltage. The bonds are dissociated, and trap centers for trapping electrons and holes are formed. Subsequently passing electrons and holes are trapped by the trap centers. The field strength distribution in the direction of film thickness becomes locally higher than the average field strength, thereby further degrading the insulating film.
The interaction between the electric field and the atomic bonds constituting the insulating film depends on the direction of the atomic bonds constituting the insulating film and the direction of the electric field. For this reason, to weaken this interaction, fewer atomic bonds are set in directions along which the interaction is strong. When the directions of atomic bonds are random like in an amorphous substance, some atomic bonds always have a direction along which the interaction is strong.
As described above, almost no relevant discussions have been made to determine design concepts of monocrystalline insulating films. Improvements of the properties of an insulating film itself depend on the formation process means. For example, a cleanest possible surface is prepared in advance, or a substrate is simply oxidized to form an oxide film thereon. Under these circumstances, it is still the case that the problem including the structure of a monocrystalline insulating film awaits a fuller explanation.